Implement PiGatewaySerial with TTY interface.

Implement MySensors Gateway for RPi with TTY
interface using PTY - pseudo terminal.

After starting it creates a MySensors GW and
PTY, in /dev/pts/ which is then symlinked to
/dev/ttyMySensorsGateway

It also handles SIGINT.

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Build the PiGatewaySerial

Signed-off-by: Tomas Hozza <thozza@gmail.com>